If a firewall is blocking the ports that ARD 2 uses, your ARD 2 admin computer won't be able to control or observe clients after upgrading them from ARD 1 to ARD 2 (version 1.2 to 2.0, for example).
If you find yourself in this situation, be sure to open the ports that Remote Desktop uses. This applies to any firewall between the administration and client systems, including NAT routers (such as an AirPort Base Station) and the Firewall feature built into Mac OS X and Mac OS X Server.
Apple Remote Desktop 2 uses some ports not used by ARD 1. You can start by opening port 5900, both TCP and UDP. For all the ports used by Apple Remote Desktop, see our complete list, which is regularly updated. For additional ARD-specific information, see Connecting through a firewall or NAT.